Holograms are an important component for a series of applications of diffractive elements in the area of optics, optical diffraction grating and/or other diffractive elements. Conventionally, the production of diffractive elements is accomplished by applying a fine structure to a flat substrate using lithographic processes such as electron beam lithography, laser lithography or photolithography, as described by H. P. Herzig in “Micro Optics Elements, systems and applications” (Taylor & Francis, 1997).
One consideration for a diffractive element is that the optical function that is to be created by the diffractive element is created with as high a degree of wavefront precision as possible. This is an important characteristic of the quality of the diffractive element itself, and thus the quality of the result of the applications that are being implemented.
Theoretically, a high degree of precision of the wavefront can be achieved according to U. D. Zeitner and E. B. Kley in “Advanced Lithography for Micro Optics” (2006), such that a very precise lateral positioning of the fine structures is warranted.
In practice, the achievable precision of the wavefront depends, however, not only on the processing method, but also on the quality of the substrates that are used. It can be said in general, that the wavefront, which is created with the help of a diffractive element, becomes correspondingly better with increasing flatness of the surface of the substrate that is used for the diffractive element. A further important characteristic of the substrate is that the flat substrate may not be deformable, but it is to have a certain rigidity. A high degree of flatness as well as also a high degree of rigidity of the flat substrate is said to be obtainable by a corresponding thickness. However, the use of thick, flat substrates represents a problem in the production of diffractive elements since, for conventional lithographic systems, only thicknesses of a few millimeters are permissible. This limitation on the thickness of the substrates is due to the handling of the substrates during lacquering and development (for the frequently used spin coating process, the substrates are rotated at a high rotational speed), the substrate mass that can still be moved with sufficient precision on the highly precise x-y tables of the lithographic systems, as well as the thermal relationships, which play a role in the structural transfer into the substrate by means of ion etching. When the flat substrates are too thick, the heat that is introduced by the ion bombardment cannot be sufficiently dissipated, which can lead to a degeneration of the resist mask.
The low thickness of the flat substrate (e.g., a few millimeters) is less significant for the lithography process, since during the lithography process, the thin flat substrate is brought to a sufficient degree of flatness by means of suitable bracketing methods such as, for example, by vacuum suction or by electrostatic suction onto an extremely flat substrate retainer. However, when separating the flat substrate from the bracketing of the respective lithographic system, the desired flatness of the flat substrate is lost. The flatness of the flat substrate that carries the fine structure is, however, a basic requirement for optic applications. Moreover, the rigidity of the flat substrate should be configured in such a way that as much as possible, no deformation of the diffractive element occurs due to external influences such as gravitation, vibration, thrusts and other influences that worsen the optical function.